This invention relates generally to seekers and more particularly to gyroscopic, spin stabilized missile seekers.
As is known in the art, seekers of the gyroscopic, spin stabilized type have been used successfully in many applications. One such system is described in U.S. Pat. No. 3,872,308 issued Mar. 18, 1975, inventors James E. Hopson and Gordon G. MacKenzie, assigned to the same assignee as the present invention. As is known, in one type of such system, a missile seeker includes a catadioptric arrangement made up of a spherical primary mirror and flat secondary mirror arranged to focus infrared energy received from an object. The primary and secondary mirrors are fixed to one another. The housing of the primary mirror is a magnet. The magnet reacts with a magnetic flux produced by adjacent, missile body mounted, motor coils, to cause the primary mirror and the attached secondary mirror to rotate as a single unit about an axis of rotation. The catadioptric arrangement is also gimballed in pitch and yaw within the missile body. The rotating catadioptric arrangement acts as a two degree of freedom gyroscope. By forming the catadioptric arrangement as a gyroscope the mass formed by the primary and secondary mirrors will maintain the axis of rotation in inertial space decoupled from the missile's body unless acted upon by a gimbal section responding to tracking boresight error signals produced by a processor.
As is also known, one missile seeker of such type includes a precession coil and a cage coil. The field produced by the precession coil drives the gimballed catadioptric arrangement in pitch and yaw within the body of the missile. More particularly, the precession coil is fixed to the body of the missile and is wrapped circumferentially about the missile's center line. The precession coil encircles, but is spaced from, the magnetic housing of the primary mirror. A sinusoidal precession coil current, having a period equal to the period of rotation of the housing about the axis of rotation, is fed to the precession coil from the processor. The precession coil current is produced to enable the gimballed catadioptric arrangement to maintain track of the target. More particularly, in response to the precession coil current, a magnetic field component perpendicular to the magnetic field of the rotating primary mirror housing, is produced by the precession coil which reacts with the rotating magnetic field produced by the permanent magnet housing to produce a torque on the housing. In response to such torque the position of the axis of rotation, in inertial space, changes. The magnitude of the rate of change in the angular position of the axis of rotation in inertial space is proportional to the magnitude of the current passed to the precession coil by the processor. Such current produced by the processor being proportional to the boresight error (i.e., the deviation between the line of sight to the target (i.e., the boresight axis) and the axis of rotation).
Also included in such seeker is a cage coil used to sense the angular deviation of the axis of rotation from the missile body's center line. The cage coil is fixed to the body of the missile and is also wrapped circumferentially about the missile body's center line in a manner similar to the precession coil so that it also encircles the permanent magnet housing of the primary mirror. The cage coil is disposed laterally along the missile body's center line and is placed adjacent to the precession coil. As the permanent magnet housing rotates about the axis of rotation, a component of the associated rotating magnetic field produced by such housing induces a sinusoidal voltage in the cage coil with a magnitude related to the magnetic flux linking to the cage coil. The magnitude of the induced voltage is proportional to the magnitude of the angular deviation of the axis of rotation from the missile body's center line. The phase of the voltage induced in the cage coil, relative to the phase of a voltage induced to a body mounted reference coil, is proportional to the angular direction of the angular deviation of the axis of rotation from a yaw axis of the missile's body. It is noted that in changing the magnitude of the current fed to the precession coil, because of the proximity of the cage coil, an unwanted voltage is induced in the adjacent cage coil. This cage coil induced voltage is proportional to the time rate of change in the precession coil current. Further, as noted above, a desired voltage is induced in the cage coil proportional to the angular deviation of the axis of rotation from the missile body's center line. The cage coil thus has induced in it a desired voltage (the voltage indicating the angular deviation of the axis of rotation from the missile body's center line) and an undesired voltage (the voltage induced in it in response to a change in the current fed to the adjacent precession coil). This undesired induced voltage thus corrupts the accuracy of the voltage induced in the cage coil.
One solution to this problem is to use a third circular coil, sometimes referred to as a caging cancellation coil, arranged to cancel the magnetic coupling from the precession coil. Achieving cancellation in this manner however, not only increases the complexity of the coil designs but also reduces the caging coil induced voltage and seriously degrades the linearity of the signal amplitude verses the angle between the axis of rotation and the missile body's longitudinal axis due to the back electromotive force (EMF) also generated in the cancellation coil.